I mean, c'mon, can there be any doubt as to my favorite gameworld as much as I idolize Gary? There is something iconic in Greyhawk; even WotC recognized that when the "default" setting for 3E was Greyhawk. And while that idea was honored more in the breach than in the observance, at least some of the Greyhawk deities made into the core rules and are the default gods for many homebrewed, DIY campaign worlds out there. Now what makes Greyhawk so iconic as opposed to a world such the Forgotten Realms (which is a close runner-up in its 1E "grey box" edition)?
First of all, Greyhawk is detailed but not too much. Even GBFR ("grey box" Forgotten Realms) has a TON of information on the NPCs of the world: stats, levels, where they're at, what they're doing, etc. Greyhawk draws a map and gives the DM names and not much else as far as that's concerned. There are plenty of hooks and history to glean and shape as the DM sees fit.
Secondly, that barebones approach can influence the tone of the game as well. With some tweaking, almost any type of setting and fantasy genre can be emulated. High Elven or Seelie court fantasy? Set a campaign at the court of Yolande of Celene and play up the otherworldliness of the Fey with Olvan cavaliers, fairies, centaurs and other sylvan peoples. Dark fantasy? Go to the Great Kigdom and run an intrigue-filled noble campaign where scions of Hextor vie with demon-summoning wizards. Is sword and sorcery more to your liking? Run a gritty campaign set in the Bandit Kingdoms or the Free Cities where the main protagonist is a barbarian from the Lands of Ice and Snow and his roguish companions (maybe a female rogue from the City of Greyhawk and a horse barbarian from the lands of the Nomads?) who become embroiled with the machinations of the Demonspawn Iuz. Arabian Nights? Ekbir and Tusmit. Chivalric fantasy? A group of knights in the Shields Land fighting against the Horned Society. Swashbuckling high seas adventure? A ship's crew sailing the briny deep from the ports of the Sea Princes. I could go on all day like this, but I think you get the point.
Third, although this doesn't really matter in the big picture, many of the classic modules are set in Greyhawk. Even if the adventures of those modules are not used as is, there's a wealth of material that is useful for the setting. The Saltmarsh series, for instance, gives structure and detail to the lands surrounding Saltmarsh (although little about Saltmarsh itself). The Sentinel and The Gauntlet, the Adlerweg adventure series, adds a pair of minor artifacts but also details another part of the Oerthian landscape. And do I even need to mention the village of Hommlet?
Greyhawk is a diverse land that is easily used for a variety of play styles and adventures. It has a level of detail that is sufficient and does not overwhelm the creative urges of the home DM. In many ways, it is either unappreciated by one group of gamers and slavishly worshipped as "Gary's" by another group. And that's a shame, because there's really much in it to be used in any AD&D campaign.